An A/D converter (analog/digital converter) is used to convert analog signals received from sensors into digital signals that can be used in a control circuit such as a microcomputer of a control apparatus, when electronic signal processing is executed depending upon the state of the signals received from the sensors.
In an anti-lock braking system of a vehicle, for example, an abnormality in a sensor or in an A/D converter should be avoided. Therefore, various countermeasures are proposed to detect an abnormality in the sensors and in the A/D converter so that the control apparatus will not fall in a fatal state.
JP 2000-151405A proposes to monitor the function of the A/D conversion by providing a monitoring A/D converter independent from the input A/D converter and comparing the results of the two A/D conversions.
This apparatus requires a plurality of A/D converters or requires an A/D converter of a circuit of a large scale. Therefore, the circuit construction and the control become complex, causing an increase in the number of parts and in the production cost. Besides, it cannot be determined which one of the A/D converters is abnormal.
FIG. 11 illustrates another prior art using a monitoring A/D converter. In this example, an electric current flowing into a motor 31 is found by measuring a voltage across both ends of a resistor 36. The electric current is found by dividing the voltage across both ends of the resistor 36 by the resistance of the resistor 36. The voltage (analog signal) across both ends of the resistor 36 is input to a terminal AIN1 of an A/D converter 34 via an amplifier 32. The voltage across both ends of the resistor 36 is further input to a terminal AIN2 of an A/D converter 35 via a monitoring amplifier 33.
In this apparatus, a microcomputer 37 compares the result of A/D conversion by the A/D converter 34 with the result of A/D conversion by the A/D converter 35 to detect abnormal condition in the A/D converters 34 and 35. In the example of FIG. 11, too, two A/D converters are used for one sensor (analog signal input), and the circuit scale becomes large.